Such a rear fairing is designed either to enclose an actuator, for example for a flap, or to envelop a structural part of said mast.
Irrespective of its use, such a rear fairing is usually close to at least one flap, which means that it must be deployable like the latter.
However, in its deployed position (tilted downward), said rear fairing frequently partly enters the cold flow of the turbine engine. The result of this is that, particularly when the thrust of the turbine engine is high and the speed of the aircraft is low, the cold flow (the speed of which is high and may even be supersonic) generates vibrations in said rear fairing, said vibrations being able to seriously damage the structure of the latter.
It will be noted that predicting the vibration level of the mast rear fairing, when it is deployed and immersed in the flow close to the engine, can currently be done only with great difficulty with the design tools available. The discovery of such a problem therefore occurs during the flight tests of the aircraft, that is to say very late in the aircraft development cycle.
In order to solve this problem, the prior art consists in:                not eliminating the vibrations, but reinforcing the structure of the rear fairing so that it withstands the vibrations. This solution leads to an increase in the weight of the aircraft, which is prejudicial to fuel consumption; and/or        redefining the aerodynamic shape of the rear fairing, which is:                    industrially costly and potentially causes a delay in the program to bring the aircraft into service, because it is necessary to redefine the tools for manufacturing the rear fairing at a very advanced stage of the development of the aircraft, and            usually a source of increased drag (and therefore fuel consumption) at cruising speed, because this change of shape leads to departing from the optimum which had initially been identified.                        